In continuous ink-jet printing, ink is emitted in a continuous stream under pressure through at least one nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the nozzle. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. Therefore, for an ink to be useful in continuous ink-jet printing, it must be able to sustain an electric charge, and must have a viscosity sufficiently low to allow ink flow through the nozzle.
Typically, the inks used for continuous ink-jet printing are liquid at room temperature. Liquid inks present various difficulties: for example, they respond differently depending upon the type of printing media used. The use of liquid ink on office papers will produce a feathered appearance because the ink penetrates and spreads into the paper following fiber lines. Liquid inks that are designed for minimum feathering still require time to set, which may limit the rate that printed pages are stacked.
The print quality usually depends on the type of paper used, which also has an effect on the drying time and on waterfastness. Although water-borne inks have been widely used, they exhibit poor waterfastness. Also, in order to prevent the ink from drying in the jet, high concentrations of humectant such as diethylene glycol have been used. This also leads to a long drying (set) time for the print on the medium and poor print quality.
Liquid inks without curable additives typically are not useful on nonporous surfaces, such and metal, glass, or plastic, because they are too prone to smearing. Further, liquid inks are very sensitive to temperature changes which influence the ink viscosity and interfacial tension, which, in turn, influence the ink interaction with the medium.
It is clear from the foregoing that major problems with liquid ink-jet inks are (1) media dependent quality, (2) poor reliability, (3) poor waterfastness, and (4) a long drying (set) time for the printed ink.
One method of solving several of the abovementioned problems is to use what is termed a hot melt ink. This ink is normally solid at room temperature. When the ink is heated, it melts to form a low viscosity fluid which can be ejected as droplets.
Hot melt ink was originally used by Berry et al. (U.S. Pat. No. 3,653,932) in electrostatically controlled continuous ink-jet printing. The ink was comprised of a waxy component which is solid at room temperature. The term "hot melt ink" defines an ink that is in a solid phase at room temperature and in a fluid phase at the operating temperature.
Hot melt inks normally comprise vehicles, such as natural waxes, resins and/or long chain fatty acids, esters, or alcohols which melt when the ink is heated to jetting temperatures. Upon jetting, heated droplets impact the print medium and immediately freeze on the medium surface. This phenomenon is advantageous in several respects in that dark, sharply defined print may be produced. This print may be slightly raised, suggesting that the print is engraved. Since the ink is solid at room temperature, during storage and shipment the colorant systems have less tendency to separate out of the ink. This has facilitated the use of various colorant systems, such as certain pigment-based systems, which would not have normally been used in liquid inks.
Despite the aforementioned advantages of hot melt inks, they have been used only sparingly to date in continuous ink-jet printing. The low molecular weight waxes and polymers typically present in hot melt inks have low polarity and show very poor solvating ability towards ionic polar material used as electrolytes in continuous ink-jet printing. To sustain the electric charge required for continuous ink-jet printing, the electrolyte ions must dissociate in the ink composition, thereby allowing ionic separation upon application of an external electric field.
U.S. Pat. No. 4,684,956 relates to the use of hot melt inks for use in drop-on-demand printing operations, although the patent contains a statement that the inks can also be used in continuous ink-jet printing. However, the ink compositions disclosed therein do not have sufficient conductivity to be usable in continuous ink-jet printing.
U.S. Pat. No. 3,653,932, which was mentioned above, discloses a hot melt ink composition comprising one or two di-esters of sebacic acid which have been esterified with alcohols of paraffins having 12 or less carbon atoms in their chains. However the resistivity of these compositions is too high for electrical charging in continuous ink jet printing.
Japanese Patent Sho 55(1980)-54368 discloses the use of resistivity control agents, such as inorganic salts, water-soluble amines, metal soaps, and metal thiocyanates. However, these ink compositions have unacceptably high volatilities at printing temperatures.
Inks containing aromatic sulfonamides have also been suggested in the art. See, e.g., U.S. Pat. No. 4,878,946. These inks also have unacceptably high volatilities at printing temperatures. Moreover, the acidic nature of aromatic sulfonamides accelerates corrosion of printhead components.
Therefore, to date there has been no hot melt ink for continuous ink-jet printing which has good conductivity, low volatility, low resistance, and acceptable viscosity.